Electronic photoflash systems and units for amateur and professional photography have become increasingly popular in recent years. A number of the early flash units required a separate power supply package, and when a photograph was being taken the flash unit emitted a given uncontrollable amount of light. The power supply typically employed a DC battery pack, a DC-to-DC converter, and a large capacitor which was charged by the battery and converter. Later automatic electronic flash units were developed based on the subject matter of Edgerton U.S. Pat. No. 3,033,988. Such units also employed a relatively large main capacitor, along with a compact DC battery source and converter for charging the capacitor. Automatic control of the light output was provided through the use of a light sensor circuit which detected when sufficient light had been received at the subject being photographed, and controlled a quench tube connected in parallel with the flashtube. With this arrangement, when sufficient light had been received, the remaining energy in the main storage capacitor was shunted past the flashtube thereby causing the flashtube to cease emitting light.
A further development, presently widely used in the photography field, was the series circuit or thyristor control circuit for automatic electronic flash units. In this system, an electronic switch is connected in series between the relatively large main capacitor and the flashtube, the series switch being turned off after sufficient reflected light had been received at a light sensor circuit. One of the benefits of the thyristor units is that the remaining energy in the main storage capacitor is conserved rather than wasted as was the case with the earlier shunt or bypass type automatic units. Examples of current commercially available thyristor flash units are, for example, the Vivitar Model 283 and Vivitar Model 285 electronic flash units. Further examples of automatic electronic flash systems employing a series circuit, and light sensor circuits therefor, are shown in U.S. Pat. Nos. 3,857,064 3,783,336, and 3,809,951, the disclosures of which are incorporated herein by reference. These patents describe several forms of series control circuits for electronic flash units, as well as forms of light sensor circuits for controlling the switching systems thereof. U.S. Pat. No. 3,591,829 also illustrate several series control circuits for flashtubes.
While both the series and shunt type automatic flash units have been made more compact and efficient, they all rely on the use of a relatively large and expensive main capacitor for supplying the energy to the flashtube. The main capacitor is charged from a set of batteries through a suitable relatively high power converter, all of which usually is contained within the flash unit housing. In the series circuit units an electronic switch having a high power rating is necessary because of the high current, such as several hundred amperes, required to be switched.
Although the aforementioned automatic electronic flash units have met with wide acceptance, they all make use of a large and expensive capacitor, which is usually referred to as a "main" capacitor. The capacity of such a capacitor typically is in the range of 500-1,000 microfarads, and the capacitor is charged to a voltage usually in the range of about three hundred to five hundred voltas. Upon discharge of the main capacitor in the flashtube, the current is in the order of two hundred to three hundred amperes for a time period usually not exceeding about one millisecond.
At least one electronic flash circuit has been proposed for obviating the need for the use of any capacitor in series with the flashtube. A system of this nature is shown in Japanese Patent Publication No. 10553/66 published June 10, 1966 based on an Application No. 6668/63 filed Feb. 9, 1963. The purpose described was to eliminate the main capacitor, which was said to comprise 50% of the entire weight of the flash unit, and to eliminate the waiting time for charging of a main capacitor. The system described employed a DC battery connected to the flashtube for igniting and supplying power to the flashtube. It is not known if this system was practical or ever commercially accepted. Furthermore, electronic flash systems have been described in which the voltage to which the main capacitor is charged can be reduced to minimize possible shock hazards or to enable different type flashtubes to be used. Examples are shown in U.S. Pat. No. 3,912,968 and No. 3,781,602. Both describe the use of a pair of charged capacitors in series with a flashtube, along with a switching arrangement for causing the voltages of the two capacitors to add to thereby provide sufficient voltage for both igniting and supplying power to the flashtube. However, all of these foregoing approaches require either a high voltage battery or a relatively large and expensive main capacitor.
The present invention pertains to an electronic flash system which does not require the usual large and expensive capacitor, but, instead, supplies power to a flashtube from a much smaller capacitor and a battery or other similar source. It is particularly applicable to automatic flash unit systems, such as of the series or thyristor type, and enables a fast flash recycle time and a large number of flashes for each battery charge. In an exemplary embodiment of the present invention, the flashtube is ionized in a conventional manner, and is ignited to commence its flash in a conventional fashion and at a conventional voltage, but this voltage is supplied by a relatively small capacitor. Then, a medium voltage battery or other suitable power supply, supplies the required medium current for a medium length of time and, in an automatic flash unit embodiment, this current is supplied through a medium size electronic switch which is controlled in a well known manner. Further, in an exemplary embodiment, a small capacitor, such as one-half to several microfarads supplies a voltage of approximately three hundred to five hundred volts for initially igniting the flashtube, for example, for the first twenty microseconds, and a sixty volt battery can supply medium current of approximately twenty amperes for maintaining and completing the flash, for such as ten milliseconds. The resulting light intensity waveform from the flashtube is lower in amplitude but longer in duration. For example, the amplitude, because of the reduced current, may be ten times less than with a conventional flash unit and typically will be approximately ten times longer such as approximately ten milliseconds instead of one millisecond, and this light intensity waveform falls within the good efficiency range of the flashtube. In addition to these benefits, the size, cost, and difficulties occasioned through the use of a large main capacitor, and the high transient currents involved, can be significantly reduced.
Accordingly, it is an object of this invention to provide an improved electronic flash system.
A further object of this invention is to provide an electronic flash system using a relatively small energy supply capacitor.
These and other objects and features of the present invention will become better understood through a consideration of the following description taken in conjunction with the drawing in which: